The production of wood which has been treated to inhibit biological decomposition is well known. Decay is caused by fungi that feed on cellulose or lignin of wood. Such organisms causing wood decomposition include: basidiomycetes such as Gloeophyllum trabeum (brown rot), Trametes versicolor (white rot), Serpula lacrymans (dry rot) and Coniophora puteana. Soft rot attacks the surface of almost all hard and softwoods, and it favors wet conditions. Most of these fungi require food and moisture, e.g., moisture contents in wood of greater than 20% are conducive to fungal growth. Dry rot is tenacious, as it can grow in dry wood. Insects are also major causes of wood deterioration. Exemplary organisms causing wood decomposition include Coleopterans such as Anobium punctatum (furniture beetle), Hylotrupes bajulus (house longhorn) and Xestobium rufovillorum (death watch beetle); hymenopterans such as termites and carpenter ants; and also by marine borers and/or wasps. Finally, termites are ubiquitous, and termite damage is estimated in the United States alone to be about $2 billion per year.
The production of wood based composite products has increased dramatically in recent years. Oriented strandboard (OSB) production exceeded that of plywood in 2000. The use of medium density fiberboard and hardboard panel products likewise has increased dramatically over the last couple of decades. However, these products are typically used in interior applications where attack from insects or decay fungi is limited, because it has been found that these products are particularly susceptible to attack by biological agents such as decay fungi and termites.
Preservatives are used to treat wood to resist insect attack and decay. The commercially used preservatives are separated into three basic categories, based primarily on the mode of application-waterborne, creosote, and oil borne preservatives. Waterborne preservatives include chromated copper arsenate (CCA), ammoniacal copper quat (ACQ, which is believed to be Copper-MEA-Carbonate and a quaternary amine), ammoniacal copper zinc arsenate (ACZA), and ammoniacal copper arsenate (ACA). Wood treated with these chemicals sometimes turns green or grey-green because of a chemical reaction between copper in the preservative and the sun's ultraviolet rays. The preservatives leach into the soil over time, especially those made without chromium, when exposed to weather. Creosote does not easily leach into soil, and it is not corrosive to metals, but it can not be painted and it leaves a dark, oily surface that has a strong odor. Oil borne preservatives are made of certain compounds dissolved in light petroleum oils, including pentachlorophenol (commonly known as “penta”), copper naphthenate, and copper-8-quinolinolate. These preservatives leave a surface that often is non-paintable, and the surface of the wood can be dark and unnaturally colored.
Modern organic biocides are considered to be relatively environmentally benign and not expected to pose the problems associated with CCA-treated lumber. Biocides such as tebuconazole are quite soluble in common organic solvents, while others such as chlorothalonil possess only low solubility. The solubility of organic biocides affects the markets for which the biocide-treated wood products are appropriate. Biocides with good solubility can be dissolved at high concentrations in a small amount of organic solvents, and that solution can be dispersed in water with appropriate emulsifiers to produce an aqueous emulsion. The emulsion can be used in conventional pressure treatments for lumber and wood treated in such a manner, and can be used in products such as decking where the treated wood will come into contact with humans. Biocides which possess low solubility must be incorporated into wood in a solution of a hydrocarbon oil, such as AWPA P9 Type A, and the resulting organic solution is used to treat wood directly. Wood treated in this way can be used only for industrial applications, such as utility poles and railway ties, because the oil is irritating to human skin.
The primary preserved wood product has historically been southern pine lumber treated with chromated copper arsenate (CCA). Most of this treated lumber was used for decks, fencing and landscape timbers. There has recently been raised concerns about the safety and health effects of CCA as a wood preservative, primarily relating to the arsenic content but also to the chromium content. In 2003/2004, due in part to regulatory guidelines and to concerns about safety, there has been a substantial cessation of use of CCA-treated products. A new generation of copper containing wood preservatives uses a form of copper that is soluble. Known preservatives include copper alkanolamine complexes, copper polyaspartic acid complex, alkaline copper quaternary, copper azole, copper boron azole, copper bis(dimethyldithiocarbamate), ammoniacal copper citrate, copper citrate, and the copper ethanolamine carbonate. In practice, the principal criterion for commercial acceptance, assuming treatment efficacy, is cost. Of the many compositions listed above, only two soluble copper containing wood preservatives have found commercial acceptance: 1) the copper ethanolamine carbonate manufactured for example according to the process disclosed in U.S. Pat. No. 6,646,147 and 2) copper boron azole. There are, however, several problems with these new copper-containing preservatives.
The soluble copper containing wood preservatives are very leachable, compared to CCA. One study has shown that as much as 80 percent of the copper from a copper amine carbonate complex is removed in about 10 years under a given set of field conditions. Under severe conditions, such as the those used for the American Wood Preserving Association's standard leaching test, these products are quickly leached from the wood. For example, we found that 77% by weight of a Cu-monoethanolamine preservative was leached from the preserved wood in 14 days. This leaching is of concern for at least two reasons: 1) removal of the copper portion of the pesticide from the wood by leaching will compromise the long term efficacy of the formulation and 2) the leached copper causes concern that the environment will be contaminated. While most animals tolerate copper, copper is extremely toxic to certain fish at sub-part per million levels. Common ranges for EC50 for copper are between 2 and 12 micrograms per liter. Another study reported following the Synthetic Precipitation Leaching Procedure. The study results showed that the leachate from CCA-treated wood contained about 4 mg copper per liter; leachate from copper boron azole-treated wood contained about 28 mg copper per liter; leachate from copper bis(dimethyldithiocarbamate) treated wood had 7 to 8 mg copper per liter; leachate from alkaline copper quaternary treated wood had 29 mg copper per liter; and leachate from copper citrate treated wood had 62 mg copper per liter. However, copper concentrations depend in part on copper concentration, and CCA had about 7% of total copper leach, the alkaline copper quaternary preservative had about 12% of the total copper leach, while the copper boron azole had about 22% of the total copper leach during the Synthetic Precipitation Leaching Procedure. Copper leaching is such a problem that some states do not allow use of wood treated with the soluble copper containing wood preservatives near waterways.
Another concern with soluble copper preservative products generally is that most preservative materials are manufactured at one of several central locations but are used in disparate areas and must be shipped, sometimes substantial distances. The cost of providing and transporting the liquid carrier for these soluble products can be considerable, and the likelihood of an extreme biological impact is very high if transported soluble copper wood preservative material is spilled or accidentally released near a waterway.
Further, unlike CCA, all of these soluble copper containing wood preservatives require a second organic biocide to be effective against some biological species. Therefore, wood preserved with these soluble copper containing wood preservatives also contains a second biocide that is efficacious against one or more particularly troublesome species. Oil-soluble biocides such as a copper (II)-sulfited tannin extract complex (epicatechins) can be dissolved in light oils, emulsified in water, and injected into the wood, as is disclosed in U.S. Pat. No. 4,988,545. Alternatively, the second biocide is often slightly water soluble or emulsified, and may be composed of a triazole group or a quaternary amine group or a nitroso-amine group, and this biocide can be simply added to the fluid used for pressure treating the wood.
One attempt to improve soluble copper containing wood preservatives was to incorporate other salts. PCT patent application WO 92/19429, published Nov. 12, 1992, in Example 2, describes a method of treating an article of prepared wood by immersing it for 20 minutes in a bath of 1800 C linseed oil containing a drying agent, or drier, of 0.07% lead, 0.003% manganese and 0.004% calcium naphthenate, 0.3% copper naphthenate, and 0.03 zinc naphthenates as an insecticide and fungicide. Others have tried alternative metal-compounds, including silver. None of these have found commercial acceptance.
Fojutowski, A.; Lewandowski, O., Zesz. Probl. Postepow Nauk Roln. No. 209: 197-204 (1978), describes fungicides comprising fatty acids with copper compounds, applied by dipping hardboard heated to 1200 C into a bath of the fungicide, also maintained at 1200 C. This is not practicable for a variety of reasons. In “A New Approach To Non-Toxic, Wide-Spectrum, Ground-Contact Wood Preservatives, Part 1. Approach And Reaction Mechanisms,” HOLZFORSCHUNG Vol. 47, No. 3, 1993, pp. 253-260, it is asserted that copper soaps, made with the carboxylic acid groups from unsaturated fatty acids of non-toxic vegetable oils, rosin, and from synthetic unsaturated polyester resins have effectiveness and long-term durability as ground contact wood preservatives for use against termites and fungal attack. These are not yet in widespread use, and are expected to have high leach rates and the bio-available fatty acids are expected to encourage some molds.
The solubility of copper preservatives can be controlled by using, for example, an oil barrier. But these oils can unfavorably change the color, appearance, and burning properties of the wood, and can be strong irritants. Oil-soaked wood containing oil-soluble biocides like chlorothalonil, e.g., utility poles, are highly resistant to leaching and biological attack, but the appearance of this wood is not acceptable for most uses. Japanese Patent Application 08183,010 JP, published in 1996, describes a modified wood claimed to have mildew-proofing and antiseptic properties and ant-proofing properties, made by treating wood with a processing liquid containing a copper salt and linseed oil or another liquid hardening composition. U.S. Pat. No. 3,837,875 describes as a composition for cleaning, sealing, preserving, protecting and beautifying host materials such as wood a mixture of boiled linseed oil, turpentine, pine oil, a dryer and 28 parts per million of metallic copper. Feist and Mraz, Forest Products Lab Madison Wis., Wood Finishing: Water Repellents and Water-Repellent Preservatives. Revision, Report Number-FSRN-FPL-0124-Rev (NTIS 1978) discloses preservatives containing a substance that repels water (usually paraffin wax or related material), a resin or drying oil, and a solvent such as turpentine or mineral spirits. Addition of a preservative such as copper naphthenate to the water repellent is asserted to protect wood surfaces against decay and mildew organisms. Soviet Union Patent No. SU 642166 describes a wood surface staining and preservation treatment, carried out by impregnating wood with an aqueous copper salt solution, followed by thermal treatment in boiling drying oil containing 8-hydroxyquinoline dye. U.S. published application 20030108759 describes injecting a copper ammonium acetate complex and a drying oil as a wood preservative. Again, oil is not favored as it can alter burning characteristics of wood, can be staining and/or discoloring, and can be an irritant. It is also difficult to work with and to inject into wood. None of the above methods of preserving wood have met commercial acceptance.
U.S. Pat. No. 6,521,288 describes adding certain organic biocides to polymeric nanoparticles (particles), and claims benefits including: 1) protecting the biocides during processing, 2) having an ability to incorporate water-insoluble biocides, 3) achieving a more even distribution of the biocide than the prior art method of incorporating small particles of the biocide into the wood, since the polymer component acts as a diluent, 4) reducing leaching with nanoparticles, and 5) protecting the biocide within the polymer from environmental degradation. The application states that the method is useful for biocides including chlorinated hydrocarbons, organometallics, halogen-releasing compounds, metallic salts, organic sulfur compounds, and phenolics, and preferred embodiments include copper naphthenate, zinc naphthenate, quaternary ammonium salts, pentachlorophenol, tebuconazole, chlorothalonil, chlorpyrifos, isothiazolones, propiconazole, other triazoles, pyrethroids, and other insecticides, imidichloprid, oxine copper and the like, and also nanoparticles with variable release rates that incorporate inorganic preservatives as boric acid, sodium borate salts, zinc borate, copper salts and zinc salts. The only examples used the organic biocides tebuconazole and chlorothalonil incorporated in polymeric nanoparticles. There is no enabling disclosure relating to any metal salts. While data was presented showing efficacy of tebuconazole/polymeric nanoparticle formulations and chlorothalonil/polymeric nanoparticle formulations in wood, the efficacy of these treatments was not compared to those found when using other methods of injecting the same biocide loading into wood. Efficacy/leach resistance data was presented on wood product material, where it was found that the nanoparticle/biocide treated wood had the same properties as the wood product treated with a solution of the biocide, i.e., the polymeric nanoparticles had no effect. Finally, it is known in the art that transport of preservative material is a large cost item, and diluents will merely exacerbate this problem.
We have discussed the problems with current systems, e.g., they add undesired oil; they increase corrosion; they are dilute; they are expensive, especially when the metal-based biocides must be combined with large quantities of organic biocides; the high copper leach rates are both a serious environmental problem in itself and will almost certainly decrease the longevity of treatment below that obtained with CCA. However, cost is a primary factor in the selection of a wood preservative. The market is accustomed to the low cost and effectiveness of CCA, and the market is not ready to bear the incremental costs of large amounts of expensive biocides and other materials such as polymeric nanoparticles.